In this section, we extend the free-space channel model to 3-D channel model proposed in [1]}, where the impacts of blockage and shadowing are taken into account and is more practical than free-space channel model. In specific, the line-of-sight (LoS) probability is given by
\(\vspace{-5.69046pt}{P_{{\rm {LoS}}}} = \frac{1}{{1 + a\exp \left( { - b\left( {\theta - a} \right)} \right)}},\) 
